1. Field of the Invention
Embodiments of the present invention generally relate to post regulation of synchronous rectifier DC-to-DC converters and, more particularly, to post regulation of existing synchronous rectifiers in such DC-to-DC converters, independent of the main control loop.
2. Description of the Related Art
In modern power supply design, DC-to-DC converters are designed to have a very high gain-bandwidth product to provide high output current slew rate and minimize the capacitance for acceptable output voltage quality. The typical isolated power supply has one control regulator that operates the primary-side switching modulator to control line and load regulation, with either single or multiple control loops on the primary and/or secondary side of the transformer. However, isolated power supplies have a practical frequency limit that is from 2 to 10 times lower than the switching frequency to maintain operational stability, thereby limiting the maximum gain bandwidth desired for the fastest load voltage transient response. The maximum switching frequency is limited by the transformer magnetic materials and designs because of core losses and leakage inductances that effect reactive and switching losses. These losses should be kept low to provide high operating efficiencies and lower heat dissipation. Therefore, the switching frequencies and the gain-bandwidth are limited and cannot be as high as desired to provide a desirably fast load transient response.
In switch-mode power supplies and other types of DC-to-DC converters, secondary post regulators that are independent of the primary-side regulator are often placed in the secondary side of the transformer for faster load voltage transient response and better load regulation. These are usually linear regulators or pulse width modulated (PWM) buck regulators of various sorts, including magnetic amplifiers (mag amps). However, all these solutions add an additional conversion stage to the power supply that adds significant losses and requires the cost, size, and complexity of another series regulator device, such as a power metal oxide semiconductor field effect transistor (MOSFET).
Overview
Embodiments of the present invention generally relate to post regulation of a DC-to-DC converter using one or more synchronous rectifiers already existing in the converter topology.
One embodiment of the present invention provides an apparatus. The apparatus generally includes a transformer having a primary winding and a secondary winding; a first switching device coupled to the primary winding for generating changes in voltage across the primary winding such that the primary winding transfers energy to the secondary winding; an output filter having an inductor and a capacitor in series; a first rectifier coupled between a first end of the secondary winding and the output filter, wherein the first rectifier is a synchronous rectifier; a second rectifier in parallel with the output filter and coupled between a second end of the secondary winding and the first rectifier; and post regulation logic configured to adjust a parameter of the synchronous rectifier based on an output of the apparatus in an effort to match a target output.
Another embodiment of the present invention provides an apparatus. The apparatus generally includes a transformer having a primary winding and a secondary winding; a means for switching coupled to the primary winding for generating changes in voltage across the primary winding such that the primary winding transfers energy to the secondary winding; a means for filtering an output of the apparatus; a first means for rectifying coupled between a first end of the secondary winding and the means for filtering, wherein the first means for rectifying is a synchronous rectifier; a second means for rectifying disposed in parallel with the means for filtering and coupled between a second end of the secondary winding and the first means for rectifying; and means for adjusting a parameter of the synchronous rectifier such that an output of the apparatus is post regulated to meet a target output.
Yet another embodiment of the present invention provides a method. The method generally includes in a first state, closing a switching device coupled to a primary winding of a transformer such that current flows through a first rectifier coupled between an output filter and a first end of a secondary winding of the transformer, through an output inductor of the output filter, and through a load in parallel with an output capacitor of the output filter, wherein the first rectifier is a synchronous rectifier and the output inductor and the output capacitor are in series; in a second state, opening the switching device such that current flows through the output inductor, the load, and a second rectifier in parallel with the output filter and coupled between the first rectifier and a second end of the secondary winding; alternating between the first state and the second state; and adjusting a parameter of the synchronous rectifier such that an output associated with the load is post regulated to meet a target output.